Calcium physiology — Which vitamin is essential to promote intestinal absorption of calcium from the gut lumen into the bloodstream?

Introduction / Context:
Maintaining calcium balance requires coordinated actions in the intestine, bone, and kidney. One vitamin upregulates key transporters and binding proteins in enterocytes, greatly enhancing dietary calcium absorption. This question evaluates your understanding of that vitamin's role in mineral homeostasis.

Given Data / Assumptions:

• Calcium absorption involves transcellular and paracellular routes.
• Hormonal regulation integrates vitamin effects with parathyroid hormone.
• Deficiency states lead to hypocalcemia and bone disease.

Concept / Approach:
Active vitamin D (calcitriol, 1,25-dihydroxyvitamin D3) binds to nuclear receptors in intestinal cells, inducing expression of calcium channels (e.g., TRPV6), the intracellular binding protein calbindin, and basolateral Ca2+-ATPase/Na+-Ca2+ exchangers. The result is enhanced transcellular calcium absorption, critical when dietary calcium is modest or physiologic demand is high (growth, pregnancy).

Step-by-Step Solution:

Verification / Alternative check:
In vitamin D deficiency, serum calcium may fall, triggering secondary hyperparathyroidism to maintain levels at the expense of bone; supplementation restores absorption efficiency and improves bone mineralization.

Why Other Options Are Wrong:

• Vitamin E: antioxidant functions; no primary role in calcium transport.
• Vitamin B12/B6: involved in hematologic and amino acid metabolism, not calcium absorption.
• Vitamin K: important for gamma-carboxylation of bone proteins but does not drive intestinal calcium uptake like vitamin D does.

Common Pitfalls:
Confusing bone mineralization cofactors (vitamin K) with the hormone-like role of vitamin D in gut absorption and systemic calcium regulation.

Final Answer:
Vitamin D.